Electric and hybrid electric vehicles employ an electric motor drive system that has lower energy costs and emits fewer pollutants than a conventional internal combustion engine (ICE) drive system. Various configurations of hybrid electric vehicles have been developed. In a first configuration, an operator can choose between electric operation and ICE operation. In a series hybrid electric vehicle (SHEV) configuration, an engine is connected to an electric motor referred to as a generator. The generator provides electricity to a battery and another motor referred to as a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. In a further configuration, a parallel hybrid electric vehicle (PHEV), an engine and an electric motor cooperate to provide the wheel torque to drive the vehicle. In addition, in a PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE. A further configuration, a parallel/series hybrid electric vehicle (PSHEV), has characteristics of both the SHEV and the PHEV.
Electric propulsion in an HEV can be performed by an electric drive system that can include a number of components, typically at least including a power conversion circuit and a motor. In this arrangement, the power conversion circuit can controllably transfer power from a power source to the motor to drive a load. A typical power conversion circuit can comprise a power source, such as a high voltage battery, and an inverter system controller (ISC) circuit, which can include a variable voltage converter (VVC) and an inverter circuit. In a typical configuration, a power source is located on what is referred to as an input side of the VVC and the inverter circuit is arranged on what is referred to as an output side of the VVC. A VVC can boost a direct current voltage provided by the battery to a higher voltage to drive the motor and improve vehicle performance. When used to boost a voltage from an input side to an output side, the converter is referred to as a boost converter.
A VVC can also be used to step down or lower a voltage from one side to another. For example, the higher voltage on a motor/generator side of a VVC can be stepped down to a lower voltage in order to charge a battery on an opposing side of the VVC. In the field of hybrid vehicles, it is common practice to charge a battery through regenerative braking, in which the mechanical energy of the wheels is converted to electrical energy by a generator, or by a motor operating as a generator, and provided to the battery via the VVC. When used to step down or reduce a voltage, the converter is referred to as a buck converter. A VVC can also operate in a pass-through mode in which transient current flows from the battery side to the inverter side, with no boost in voltage.
Generally it is preferable that the output voltage VO on the inverter or motor/generator side of the VVC should remain higher than the input voltage VI on the battery side of the VVC in a power conversion circuit for a hybrid vehicle. When the output voltage VO drops below VI, control over the VVC can be lost, causing the system to become unstable. Such a voltage drop can occur in a hybrid electric vehicle when a motor makes a sudden power demand, or during active motor damping (AMD) braking operations. Prior art methods of answering the intrinsic instability problem of some VVC operations included methods that focused on avoiding the conditions that could trigger VVC instability. For example, some solutions required that circuits be designed with finely tuned control parameters. However, because there can be disparity in circuit component characteristics, for example, capacitances can vary by as much as 20%, the control parameters had to be customized for individual circuits. Circuit customization can be time-consuming and expensive, prohibitively so for systems manufactured at mass-production facilities.
At the power supply side of a VVC, is typically an inductor configured to store energy to be transferred across the VVC. In general, in prior art circuits current flows through the VVC inductor and the VVC switches regardless of whether the VVC is operating in a buck, boost or pass-through mode. This multiple-mode current flow requirement controlled the inductor specifications, the heat dissipation design, the rate at which energy could be transferred, the power losses of a VVC, and the maximum amount of power that could be delivered by a VVC. In addition, the power flow limitation often worsened as the voltage of the high voltage battery decreased.